A new paper says it’s not clear:
Scientists have long thought that there was a direct connection between the rise in atmospheric oxygen, which started with the Great Oxygenation Event 2.5 billion years ago, and the rise of large, complex multicellular organisms.
That theory, the “Oxygen Control Hypothesis,” suggests that the size of these early multicellular organisms was limited by the depth to which oxygen could diffuse into their bodies. The hypothesis makes a simple prediction that has been highly influential within both evolutionary biology and geosciences: Greater atmospheric oxygen should always increase the size to which multicellular organisms can grow.
It’s a hypothesis that’s proven difficult to test in a lab. Yet a team of Georgia Tech researchers found a way — using directed evolution, synthetic biology, and mathematical modeling — all brought to bear on a simple multicellular lifeform called a ‘snowflake yeast’. The results? Significant new information on the correlations between oxygenation of the early Earth and the rise of large multicellular organisms — and it’s all about exactly how much O2 was available to some of our earliest multicellular ancestors.
“The positive effect of oxygen on the evolution of multicellularity is entirely dose-dependent — our planet’s first oxygenation would have strongly constrained, not promoted, the evolution of multicellular life,” explains G. Ozan Bozdag, research scientist in the School of Biological Sciences and the study’s lead author. “The positive effect of oxygen on multicellular size may only be realized when it reaches high levels.”“Did Earth’s Early Rise in Oxygen Support The Evolution of Multicellular Life — or Suppress It?” at Georgia Tech Earth and Atmosphere Sciences (May 14, 2021)
The paper is open access.
So if the value of oxygen is dose-dependent, life probably had even less time to work with to become large and complex.